1. Field of the Invention
The present invention is directed to a method and system for the process of reactively converting a liquid fuel into a gasified stream. More particularly, the method and system of the present invention provide a novel means for converting the liquid fuel into a gas by partial oxidation and steam reforming. In addition, when fuels containing sulfur are used, a device according the present invention can be employed to provide de-sulfurization.
2. Brief Description of the Related Art
Gasification of liquid fuels typically comprises use of a vaporizer. Vaporization of liquid fuels (e.g., alcohols, hydrocarbons) typically is achieved by indirectly supplying heat into a stream of liquid fuel via heat exchange with a hot wall. One disadvantage of this method is that the rate of vaporization is limited by the rate of heat transfer such that relatively large surface area is required for fuel vaporization. Another disadvantage of this method, especially for vaporizing long chain hydrocarbons, is that heating the fuel stream to the vaporization temperature tends to cause fuel decomposition and formation of deposits. More specifically, coke formation is problematic. Moreover, preventing deposits from forming within the fuel passages in the nozzle during steady state operation due to heat-up of the nozzle from the downstream hot zone is challenging.
Another known method for gasification of a fuel stream comprises mixing atomized fuel with a hot gas such as superheated steam that supplies the heat required for fuel vaporization and prevents coke formation. However, the large amounts of superheated steam required in this method result in a large heat load for steam production.
Spray methods for atomization of liquid fuels known in the art include air-blast or pressure atomizers, ultrasonic and electrospray atomizers. These spray systems are capable of providing a uniform distribution of atomized fuel across the entrance of the catalyst bed. Such atomizers may include a co-flow of air that allows mixing of the fuel and oxidizer. However, very fine and uniform droplet size along with homogeneous fuel-air distribution, required to avoid coke formation and obtain temperature/mixture uniformity in the reactor, is difficult to achieve in practical systems.
Ignition devices, such as a spark or glow plugs, are widely used to ignite fuel-oxidizer mixtures at startup. These devices often are subject to failure due to the high operating temperatures by virtue of their location required for ignition.
Monoliths are commonly used catalyst substrates for the gasification of liquid fuel. Fuel oxidizer mixture inhomogeneities are usually detrimental to these substrates as they lead to localized lean or rich zones respectively causing hot spots or carbon precipitation regions. Since there is no opportunity for these zones to re-mix within the long, separated channels of a monolith, these substrates are particularly vulnerable. In addition, carbon precipitation is favored in monoliths due to the boundary layers that develop in these substrates.
Combustion of liquid fuels in fuel cell or internal combustion engine systems poses significant problems, especially for fuels with high aromatic content and wide boiling point distribution. This can be attributed to the propensity of the heavier aromatic compounds in the fuel to form deposits or coke when vaporized at high temperatures.
Liquid hydrocarbon fuels such as gasoline, kerosene or diesel may be used with high temperature solid oxide fuel cells (“SOFC”) to directly produce electric power. For these fuel cells, the choice of fuel is not limited to pure hydrogen as is the case for low temperature proton exchange membrane (“PEM”) fuel cells. Conversion of the hydrocarbon fuel into gaseous mixture containing syngas, though, is required before the fuel may be fed to the SOFC. Furthermore, removal of sulfur normally contained in the fuel prior to feeding to the SOFC is needed.
These and other known methods and systems for gasification of liquid fuels, together with their associated disadvantages, are described further in U.S. patent application Ser. No. 10/902,512 filed on Jul. 29, 2004; which is incorporated herein in its entirety.
Gasification and pre-reforming of liquid fuel would resolve many of the issues noted above with respect to the prior art. Accordingly, there is a need for a pre-reforming reactor capable of operating with a range of liquid fuels. It is therefore an object of the present invention to provide a pre-reforming reactor for partially oxidizing and cracking the heavy components of the fuel. The pre-reformed fuel subsequently can be further reformed or combusted to power fuel cell systems, internal combustion engines, burners, and other known devices.
It is therefore another object of the current invention to provide a catalyst substrate that facilitates mixing of the stream flowing therethrough, for example a substrate having plurality of voids in random order and short channels extending in the downstream direction the length of which is similar to the channel diameter. Such a configuration results in a comparatively high conversion rate of the reactants to the desired products and minimizes break through of unreacted fuel.
It also is an object of the current invention to provide a catalytic reactor for the gasification of liquid fuels comprising a catalyst that yields partial oxidation products, such as CO and H2. This results in a higher level of fuel conversion for the same amount of added air and produces hydrogen-rich gas directly from the gasifier reactor. It is a further object of the current invention to provide a method whereby steam or atomized water and/or CO2 may be added to the fuel/air stream to adjust the amount of hydrogen in the product stream. It also is a further object of the current invention to provide a method whereby no external pre-heating of either air or fuel is required.
Lastly, it is a further object of the present invention to provide de-sulfurization of the fuel in the liquid form when required by a particular application of the gasification system taught herein.